1. Field of the Invention
The present invention relates to an externally pressurized air bearing spindle device, supporting a spindle or main shaft without making contact by an externally pressurized air bearing, which may be used as a spindle device for drilling machine, precision machine tool, electrostatic painting machine, etc.
2. Description of Related Art
The externally pressurized air bearing supports the main shaft without making contact therewith, and is hence characterized by small friction loss and excellent precision of motion guide. It is hence used as the main shaft bearing for high speed spindle or precision spindle of drilling machine, precision machine tool, electrostatic painting machine, etc.
FIG. 6A and FIG. 6B show an example of a conventional externally pressurized air bearing spindle using an externally pressurized air bearing. This externally pressurized air bearing spindle is of air turbine system, that is, plural recesses 21a1 are provided in the outer circumference of a thrust plate 21a of a main shaft 21, and plural turbine nozzles 22c opened in the tangential direction are provided at positions confronting the recesses 21a1, compressed air supplied from a turbine air feed port 22a through turbine air feed passages 22b and 22d is blown to the recesses 21a1 of the thrust plate 21a from the turbine nozzles 22c in the tangential direction to rotate the main shaft 21. The main shaft 21 driven in this manner is supported by a first bearing portion X' and a second bearing portion Y' without making contact. The turbine nozzles 22c and the turbine air feed passage 22d in the circumferential direction are formed in a turbine nozzle member 22, and the turbine nozzle member 22 is fixed inside of a bearing housing 25 of the second bearing portion Y'.
The first bearing portion X' is composed of a cylindrical bearing housing 23 for composing its outer wall, and a bearing sleeve 24 fitted to the inside of the bearing housing 23 by proper means such as shrinkage fitting, press fitting or adhesion. Inside of the bearing sleeve 24, there is a journal bearing surface 24a confronting the surface on the outer diameter of the main shaft 21 with a small journal bearing clearance, and at the rear end of the bearing sleeve 24, there is a thrust bearing surface 24b confronting the leading end surface of the thrust plate 21a of the main shaft 21 across a small thrust bearing clearance. On the bearing sleeve 24, moreover, there are plural fine bearing nozzles 24c, 24d opened to the thrust bearing surface 24a, and plural fine bearing nozzles 24e opened to the thrust bearing surface 24b. The bearing nozzles 24c, 24d, 24e are arranged in a same cross section orthogonal to the axial line of the bearing sleeve 24.
The second bearing portion Y' is composed of a cylindrical bearing housing 25 coaxially jointed to the rear end of the bearing housing 23, and a bearing sleeve 26 fixed to the inside of the bearing housing 25 by proper means such as shrinkage fitting, press fitting or adhesion. At the leading end surface of the bearing sleeve 26, there is a thrust bearing surface 26b confronting the rear end surface of the thrust plate 21a of the main shaft 21 with a small thrust bearing clearance. In the bearing sleeve 26, moreover, there are plural fine bearing nozzles 26c, opened to the thrust bearing surface 26b. The bearing nozzles 24c are arranged in a same cross section orthogonal to the axial line of the bearing sleeve 26.
As shown in FIG. 6B, joining of the bearing housing 23 of the first bearing portion X' and the bearing housing 25 of the second bearing portion Y' is achieved by mutually joining the junction surfaces of fixing parts 23a, 25a provided for each housing, and inserting a coupling bolt 27 from the side of the fixing part 25a into screw holes 23a1 provided in the fixing part 23a, and tightening.
By the compressed air flowing into each bearing clearance from the bearing nozzles (24c, 24d, 24e, 26c) of the bearing sleeve 24 and bearing sleeve 26, the displacement of the main shaft 21 in the radial direction and thrust direction is suppressed. This compressed air for bearing is supplied from a bearing air feed port 28, passes through radial air feed passages 29, 30, and axial air feed passages 31, 32, enters circumferential air feed passages 33, 34, 35 communicating the bearing nozzles (24c, 24d, 24e, 26c) in the circumferential direction, and flows into bearing clearances through bearing nozzles (24c, 24d, 24e, 26c). The compressed air flowing into the bearing clearances reaches the bearing end through the bearing clearances, and is discharged outside the spindle directly, or through exhaust passages 36, 37. By the pressure distribution of the compressed air occurring in the bearing clearances, the main shaft 21 is supported without contacting the journal bearing surface 24a or thrust bearing surfaces 24b, 26b.
In this way, in the conventional spindle, the first bearing portion X' and second bearing portion Y' for supporting the main shaft 21 without contacting therewith are respectively constructed of the bearing housings (23, 25) and bearing sleeves (24, 26) integrally fitted together, and the main reason is that the circumferential air feed passages (33, 34, 35) for communicating the bearing nozzles (24c, 24d, 24e, 26c) in the circumferential direction cannot be formed if the bearings are built in a single cylinder structure. It is also of the same reason that the turbine nozzle member 22 is provided.
In the field of machine tools mentioned above, recently, in order to improve the productivity, there is a tendency to multiple dimensions and higher speed for positioning of spindle, and as one of the elements for such purpose, reduction of weight of the spindle is an important subject. For example, in the electrostatic painting machine, in order to perform painting more flexible and finely, there is an increasing demand for mounting the spindle on a multiple-joint robot, instead of the conventional reciprocating table, and the reduction of spindle weight will be a great merit from the viewpoint of limitation of the load capacity of the robot.
In this kind of spindle, on the other hand, the dimensions, shape and material of the main shaft are often determined by the functional requirements, such as spindle load capacity, rigidity, elongation by thermal deformation, and wear resistance, and weight reduction of spindle mainly depends on weight reduction of the bearing portions. As the means for such purpose, it may be considered to compose the bearing, especially the bearing housing, by using material of low specific gravity, for example, aluminum alloy and other light metal material, ceramics, synthetic resin, graphite, and other nonmetallic materials (generally the bearing housing is made of stainless steel, and the bearing sleeve is made of bronze alloy), or to reduce the wall thickness of the bearing housing or bearing sleeve.
However, in the conventional spindle as shown in FIGS. 6A and 6B, the bearing portion is an integral structure combining the bearing housing and bearing sleeve, and as compared with the single cylinder structure of same thickness, the rigidity is small structurally. Accordingly, if various means for weight reduction are directly applied to the conventional spindle, due to lowering of rigidity of the bearing portions, the entire spindle may oscillate by acceleration due to imbalance of the main shaft during operation, and the run-out of the main shaft is magnified, and sufficient precision of rotation may not be obtained.
Besides, the conventional spindle is complicated in the structure of the bearing portions, requiring many processes in manufacture of bearing housing and bearing sleeve, and it was generally expensive.